On the subject of an increase once more (4,5)
I believe the answer is:
Here is my best explanation:
'once more' is the definition.
('over again' can be a synonym of 'once more')
'on the subject of an increase' is the wordplay.
'on' becomes 'over' (I've seen this in other clues).
'the subject of an increase' becomes 'again' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
(Another definition for over again that I've seen is "Once more".)